This invention pertains to cementing of oil and gas well and, more particularly, to a reverse circulation equipment tool and process.
In the construction of oil and gas wells, a well bore is drilled into one or more subterranean formations or zones containing oil and/or gas to be produced. The well bore is typically drilled utilizing a drilling rig which has a rotary table on its floor to rotate a pipe string during drilling and other operations. The drilling rig may also have a top drive mechanism for rotating the pipe string which is integral with the traveling block of the rig in addition to or instead of a rotary table.
During a well bore drilling operation, drilling fluid, also referred to as drilling mud, is circulated through nozzles on the drill bit and upwardly back to the surface through the annulus between the walls of the well bore and the drill string. The drilling mud is typically either water-base or oil-base and contains a variety of components. The primary functions of the drilling mud are to lubricate the drill bit, to transport rock cuttings to the surface and to maintain a hydrostatic pressure in the well-bore sufficient to prevent the intrusion of formation fluids and thereby prevent blowouts.
Following drilling, a casing or pipe is cemented in the well-bore to prevent caving in of the hole and to segregate the formations penetrated. Typically, after a well for the production of oil and/or gas has been drilled, a casing will be lowered into and cemented in the well. The weight of the casing, particularly with deep wells, creates a tremendous amount of stress and strain on the equipment used to lower the casing into the well. In order to minimize that stress, floating equipment, such as float shoes and/or float collars can be used in the casing string.
The float equipment typically has a valve affixed to the casing which allows fluid to flow down through the casing, but prevents flow in the opposite direction. Because upward flow is obstructed, a portion of the weight of the casing will float or ride on the well fluid to reduce the amount of weight carried by the equipment lowering the casing into the well. Once the casing is in position, cement is pumped down through the inner diameter of the casing, through the valve and into the annular space between the outer diameter of the casing and the well bore. After the cement job is complete, the valve keeps the cement below and behind the casing string.
The float equipment is typically fabricated with a check valve in an outer sleeve which is screwed into a casing string. The valve can be affixed by filling the annulus between the valve housing and the outer sleeve with a high compressive strength cement to form a cement body portion.
As discussed above, in running a string of well casing in a well bore, it is often the practice to cause the well fluid to sustain a portion of the weight of the casing string by floating the string in the well fluid. The well fluid is ordinarily prevented from entering the casing string by an upwardly closing check valve, which later prevents back flow of the cement slurry, pumped down and around the casing string by conventional cementing.
A shoe on the lower end of a string of casing can be provided in order to guide the casing through the well bore and protect the casing from damage by contact with the wall of the well bore. Sometimes the openings on the sides of the shoe help jet the well bore walls for improved cleanings. In some circumstances, it has been the practice to use a type of shoe with a valve which either totally excludes well fluid from the interior of the casing string or which permits a limited amount of fluid to enter the string. Shoes of this type are often referred to as float shoes and differential flat shoes, respectively.
It is the usual practice to cement the casing in place to prevent migration or channeling of water or other fluids along the outer side thereof. Before cementing, annular space between the outside of the casing and the wall of the borehole is the conditioned for cementing by pumping conditioning fluid down the casing. The conditioning fluid flows radially outwardly from the bottom of the casing and passes-upwardly through the annular space where it entrains and carries rock cuttings and other to the surface. The conditioning fluid usually comprises drilling mud followed by thinner drilling mud of lesser viscosity which can more easily be displaced during cementing.
Conventional cementing techniques involve displacing cement slurry down through the bore of the casing and out a shoe on the bottom thereof so that the cement fills the annulus between the casing and the well-bore wall. A sufficient volume of slurry is displaced so that the top of the cement in the annulus extends inside the previously cemented string of casing. The casing is made up of a number of cylindrical sections or joints which are passed down the hole in sequence and which are screwed together end-to-end. As one moves down to lower depths the diameter of the casing is reduced. It often is the practice to run a number of lengths of casing of constant diameter into the hole, then to pump cement down the casing, out of the end of the casing and upwardly into the gap between the borehole wall and the outer wall of the casing in order to seal the casing and hold it in place. When the cementing operation is completed, further cylindrical sections of casing of reduced diameter can be passed downwardly through the first casing section. The casing sections (joints) are screwed together so that they extend downwardly from the first section. These procedural steps can be repeated with reducing diameter casing sections. A shoe or float can be placed at the bottom end of the leading casing section. Furthermore, an internal collar can be secured part-way down the length of the leading casing sections.
In conventional primary cementing, cement is forced down the bore of the casing, through an aperture in the guide shoe at the bottom of the casing and up the annulus between the casing and the well-bore to the desired level. One or more float valves are installed in the casing to prevent back flow of the cement into the casing from the annulus if pressure in the casing is reduced and because the density of the cement slurry is normally higher than the density of the displacing mud in the casing. A float valve may be in the form of a collar or as an integral part of the guide shoe. The closed float valve or valves seal the bottom of the casing and prevent fluids in the well-bore from filling it when the casing is lowered into the well-bore. The casing float provides buoyancy in the casing and can reduce total weight supported by the derrick.
After the casing is in place in the well-bore, a bottom cement plug can be pumped before the cement in order to displace any fluid in the casing. The bottom plug can be pumped downwardly through the casing to seat above the uppermost float valve. Thereafter, the pressure is increased in the casing, a diaphragm in the bottom plug is ruptured, and cement flows through the bottom plug, opening the float valve or valves by overcoming the biasing mechanism of the valve. The cement travels to the well-bore annulus. A solid top plug follows the cement and is pumped down by through the casing bore to seat on the bottom plug, at which point the back pressure from the cement in the casing below the float valve, and in the well-bore annulus is supposed to close the valve. When the top plug lands on the bottom plug, the surface pressure increase indicates the end of the cement job.
In conventional cementing, the bottom plug with a rupture disc or the like is usually run ahead of the cement column in the casing. A displacement plug or top plug can be run at the upper end of the column to separate the cement and the displacement fluids. When the bottom plug reaches the shoe at the bottom end of the casing, pressure is used to rupture the disc so that cement slurry can be pumped out of the casing and upwardly into the lower end of the annulus. When the top or displacement plug reaches the shoe, most all of the cement slurry will have been pumped into the annulus. Once the cement has set up or hardened, perforations are shot at one or more intervals in the casing in order to communicate hydrocarbon-bearing formations with the bore of the casing so that the well can be placed on production.
Although the conventional cementing technique has been used for many years, it has a number of shortcomings. The process is time consuming because the cement must be pumped all the way to the bottom of the casing and then back up into the annulus. Expensive chemicals often are used to retard setting of the cement. These factors make conventional cementing a very high cost process which adds considerably to the total completion costs of a well.
One problem which arises when carrying out conventional cementing, is the problem of "free falling" of the cement which occurs during the initial pumping of the cement slurry down the casing. Particularly with larger size casings, the cement slurry falls freely out of control.
There are other drawbacks to conventional cementing which have long plagued oil and gas well production. Prominent among these is that in pumping the cement downwardly to the bottom of the casing and then upwardly into the bore hole annulus to the desired height for the cement column, a considerable period of time is involved. This often results in the use of large concentrations of cement retarder. These conditions can be aggravated by the relatively high temperatures in the bore hole and the water loss from the cement to the formation. In extreme cases, the cement may even set before reaching its destination.
Another drawback in conventional cementing is the weight of the cement, which is heavier than the drilling mud. As the cement travels downwardly in the casing, considerable weight is placed on the casing string. A further problem in conventional cementing, is in pumping the cement upwardly through the annulus from the bottom of the casing or from discharge ports in the side of the casing. In conventional cementing, the pump pressure must be sufficient not only to overcome the resistance to flow of the fluid cement, but also to overcome the weight differential between the cement outside of the casing and the mud inside. These excessive pressures in many cases contribute to the failure in obtaining an annular cement column of adequate height because under high pumping pressures the well walls often fracture, causing loss of the cement to the formation before the cement has sufficiently set.
It is important for the cement to form a strong, continuous annular wall or sheath which bonds the casing to the wall of the well bore. The cement should completely surround the circumference of the casing and should extend uniformly through the vertical length of the annular interval cemented. If the cement is weak, or if any voids are left therein, several undesirable consequences can result. A poor cementing job will not effectively segregate the formations penetrated by the well-bore, and unwanted communication between the formations may occur, sometimes resulting in the production of unwanted fluids. Also, production fluid from a petroleum bearing formation may flow through channels in the cement and into another formation, where it is lost. This is especially disadvantageous when the other formation contains an aquifer. Contamination of the hydrocarbon-bearing formation itself can also occur, such as when salt water channels through the cement and flows into the hydrocarbon-bearing formation. Also, an unsatisfactory cementing job can cause the loss of treatment fluids which are pumped down the well to stimulate production of oil or gas.
In reverse circulation cementing, cement is pumped downwardly into the annulus between the casing and wall of the well bore, without pumping the cement downwardly through the interior of the casing. This has been accomplished by different techniques.
Following the running of the casing, drilling mud is circulated by the mud pump in the conventional manner to pass downwardly through the casing where the mud is discharged through the float shoe. During this initial circulation, the casing can be reciprocated and/or rotated to abrade the wall of the bore hole as well as to remove any accumulated mud cake, which will be carried out along with cuttings and debris by the circulating drilling mud. When an inspection of the rotary mud at the pit indicates that the well is substantially clean, the circulation of mud is then reversed. Once the reverse circulation cycle has been established, the well is ready for cementing. The mud pump is stopped, a spacer fluid is pumped, and a cement slurry is pumped from the cement trucks into the annulus driving the mud ahead of it so that the mud continues to be discharged from the upper end of the casing back to the mud pit.
The reverse cementing operation is continued until such time as the cement has entered the shoe and has begun to flow upwardly into the casing. The moment when this occurs can be determined by observation of the reduction in volume and velocity of mud returns measured at the surface, as well as the variations in pressure registered on pressure gauge. This measurement can be facilitated by providing in or near the shoe a restricted orifice which will cause a more pronounced flow change to appear at the time the cement enters the pipe if the cement is of greater viscosity than the mud. Also, a conventional weight indicator which displays the approximate weight of the pipe and its contents may be used in the suspension system for the casing, in which case the entry of the cement will be reflected by the change in weight of the casing. In the situation where only a partial cementing is contemplated, i.e., where the cement column will be localized somewhere between the top and the bottom of the bore hole, the amount of cement is theoretically calculated in advance in the same manner as is done conventional cementing practices.
The following alternative mode of operation can also be used in reverse cementing some wells. After the initial mud circulation downwardly in the casing and upwardly in the annulus has been established and it appears from examination of the mud being discharged into the mud pit through the line, that the well is substantially clean, the mud pump is stopped and the mud valve is closed. Conditioned mud valve now is opened permitting the conditioned mud to be pumped through the feed line into the upper end of the casing in place of the heavier drilling mud previously circulated. The amount of conditioned mud can vary as desired, depending upon the condition of the well walls, the bottom hole pressure encountered, etc. It often is desirable to pump drilling mud until the reverse circulation is well established and then to shut off the mud pump and begin reverse cementing. The cement slurry preceded by the spacer fluid can immediately be pumped from the cement trucks into the annulus.
The weight differential between the column of fluid inside the casing and that outside assists in initiating and maintaining reverse cement circulation so that less pumping pressure is required and the bottom hole pressure is less than conventional cementing. Additionally, the friction pressure at the bottom of the hole during reverse cementing is much lower then conventional cementing. The cement can be mixed and fed faster, with less pumping power. Because of the lower pressures in reverse cementing, there also is less tendency for the mud or cement to be forced laterally of the bore hole into weak or unconsolidated formation zones near the bottom of the hole. Furthermore, in the event difficulty is encountered due to loss of circulation as sometimes happens in practice, reverse cementing greatly facilitates re-establishment of circulation.
When the walls of the bore hole to be cemented are relatively clean to begin with, or are relatively easy to clean, it is possible to eliminate the preparatory step of circulating conditioning fluid comprising drilling mud downwardly into the upper end of the casing to establish circulation downwardly in the casing and then upwardly in the annulus for the purpose of removing rock cuttings, mud cake, and other debris. When cleaning of the bore hole is completed or not required cementing operation with the circulation of the mud in the reverse direction, can commence by pumping downwardly in the annulus and thereafter upwardly in the casing as has been described. After circulation in this direction has been established, the pump supplying the mud through the line is stopped and the calculated amount of cement slurry is fed after the spacer fluid into the annulus so that the cement travels downwardly to the desired destination. The casing can be reciprocated during the placing of the slurry to abrade cement cake from the walls and maintain an open path for the slurry.
One reverse cementing system has a cementing shoe on the lower end of the casing with a normally closed valve element that can be locked open, when the casing string is run. A check valve is positioned in the casing several joints above the cementing shoe and has a normally open flow ports with downward facing valve seats. Well conditioning fluids comprising drilling mud are pumped down the casing, through the check valve and the cement shoe, and into the annulus so that the annulus can be cleaned up prior to cementing. Then a blowout preventer is closed at the surface and cement slurry is pumped through a line into the annulus. The column of cement can be preceded by a fluid spacer which separates the slurry from the well conditioning fluids, and an injector is used to place a plurality of balls or ball discs in the annulus at the front of the cement column or at the top of the mud spacer. The spacer and slurry pass downward into the annulus between the casing and the borehole wall and then over the lower end of the casing via the locked-open valve in the cement shoe. When the balls or discs reach the valve seats in the check valve, they lodge in the valve seats to prevent upward flow therethrough. When this occurs a positive indication is given at the surface in the form of a pump pressure increase and/or cessation of mud flow. The cementing job is then complete, and the pressure can be bled off at the surface. It has been suggested to drop a test ball down the casing on an upwardly facing valve seat on the check valve so that internal pressure can be applied to the casing string to test for leaks.
Reverse circulation cementing is currently being used in the field almost exclusively in relatively shallow wells. In these applications, cementing is performed by taking returns through an inner string run inside the casing after getting the casing to bottom. The inner string stings into a tool at the bottom of the casing. The valve in the tool closes after the inner string is un-stung from the tool after the end of the cement job.
Some prior reverse float equipment in shallow well applications have used a retainer at the bottom of the casing, in conjunction with the use of an inner string. Returns are taken during the job through the inner string. At the end of the job, the inner string is pulled from the tool to close the valve at the bottom of the casing, allowing the cement to set without having to apply pressure to the casing. For deeper applications, running an inner string is not operationally easy and in many cases undesirable. Therefore, new float equipment is needed to be able to use reverse circulation cementing for deeper applications without the use of an inner spring and without the application of pressure to the casing after the end of the cement job.
It is therefore, desirable to provide an improved reverse circulation float equipment tool and process, which overcomes most, if not all, of the preceding problems.